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Ref ID : 7576

Hans-Dieter Gortz and Josef Dieckmann; Life Cycle and Infectivity of Holospora elegans Haffkine, a Micronucleus-specific Symbiont of Paramecium caudatum (Ehrenberg). Protistologica XVI(4):591-603, 1980

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The life cycle of Holospora elegans Haffkine, a bacterium which grows and multiplies exclusively in the micronucleus of Paramecium caudatum, is described. The bacterium occurs in two morphologically distinct forms, a short reproductive form (length 2 µm) which is characterized by a homogenous cytoplasm, and a conspicuous long form (length 10-20 µm) which, when observed with the dark phase-contrast microscopy, is composed of a large refractile area with a pale tip and an equally large dark looking area. This long form is the infectious form of H. elegans. The short form which can often be found dividing cannot infect other paramecia. When it is taken up into a food vacuole it is digested. The infectious form is not digested but quickly starts to change its appearance: The retractile part decreases in length and becomes pale. Then the bacterium leaves the food vacuole and enters the micronucleus. During this process the bacterium remains surrounded by membranes. In the micronucleus it constricts at 3 to 5 points giving rise to several bacteria of the reproductive form. After several days of rapid multiplication the infectious form may be developed by outgrowth and differenciation from the short form. The infectious form may leave the micronucleus during division. Bacteria of this form are found to be scattered in the cytoplasm of newly divided paramecia from where they are released in an unknown way into the external medium. The whole life cycle is well adapted to the physiological conditions of the host paramecium. Outside paramecium the infectious form keeps its infectively for at least three weeks at low temperature and in non-dividing paramecia it is maintained in the micronucleus. However, with respect to the mode of development, the cytological appearance, and the missing resistance against heat and dessication, the infectious form differs from typical bacterial spores. When the host paramecia grow the infectious form may develop into the reproductive form inside the same micronucleus. The infection is highly specific to P. caudatum, but strains belonging to different syngens have been successfully infected with H. elegans. It was found that it cannot infect strains of the species of P. aurelia from which a number of stocks have been tested. The infection of other ciliates was equally impossible. Under laboratory conditions H. elegans harms its host. Micronuclei swell after infection. They may divide unequally and even be lost completely. Paramecia without a micronucleus normally die. Fission anomalies may also lead to more than one micronucleus per cell. The fission rate of infected P. caudatum regularly drops. However, infected cells may outgrow H. elegans. They may later become infected again. H. elegans has been maintained without special precautions in stock cultures for several years and we have always found at least 80% of the paramecia infected. An equally high rate of infection was also maintained in the natural pond, from which the infected cells have been isolated. Therefore, being infected with H. elegans may provide paramecium with a selective advantage. This could be due to a possible protection against infections with pathogenic bacteria, like the protection against a secondary infection with the macronucleus-specific H. obtusa.